Modeling Compound Flooding in South Louisiana

Compound flooding from coastal storm surge and tropical rainfall fluvial flows during hurricane or extreme tropical events can be disastrous. Traditionally, separate numerical models have been used to analyze coastal storm surge (ADCIRC) and riverine flows (HEC-RAS) without providing a direct way to link these processes together within the models. Recent updates to HEC-RAS model provide new capabilities for coupling ADCIRC coastal surge data with HEC-RAS riverine models as a spatially and temporally varying downstream boundary condition.

This spatiotemporal data coupling approach was applied by HDR as part of the Louisiana Watershed Initiative (LWI) funded by the Louisiana Department of Transportation and Development (LA DOTD). The coupled modeling focused on three Hydrologic Unit Codes (HUC) 8 subregions within Region 5 Watershed; Mermentau, Vermillion and Atchafalaya. These HUC’s are characterized by river-dominated regimes that transition to tidal- and wave-dominated zones along the coastal estuarine wetlands before discharging into the Gulf of Mexico.

Four historical tropical storm flood events and four non-tropical flood events were simulated within three HUC8 HEC-RAS watershed models in South Louisiana. The inland forcing for the models included hindcast rainfall and upstream discharge measured by gauges. Wind field data from the coastal Gulf-scale models were extracted and spatiotemporally applied to the large open water features within the watershed domains. The modeling synthesized best available elevation data from LiDAR and bathymetric surveys to develop 2-dimensional mesh modeling grids.

The model boundary conditions and the model results for water level and discharge were compared with available NOAA and USGS gauge data to assess model skill. The location of the model domain boundary was tactically positioned and adjusted as needed to optimize respective coastal/riverine modeling skills. Results on coupling water levels in the transition zone between riverine and tidally dominated regimes, referred to herein as the Coastal Transition Zone (CTZ), highlighted the benefits for maximizing the skills of both coastal and riverine models to model and validate compound flooding events. The LWI models were delivered to the DOTD and are intended to be used by local communities, i.e., floodplain managers, to better guide local flood protection and assess resiliency projects.